1. Field of the Invention
This invention relates to bus arbitration within computer systems and, more particularly, to a computer system having an improved bus arbiter for arbitrating bus accesses of a CPU, real time DSP hardware, and other system resources.
2. Description of the Relevant Art
Computer architectures generally include a plurality of devices interconnected by one or more buses. For example, conventional computer systems typically include a CPU coupled through bridge logic to a main memory. The bridge logic also typically couples to a high bandwidth local expansion bus, such as the Peripheral Component Interconnect (PCI) bus or the VESA (Video Electronics Standards Association) VL bus. Modern local bus standards such as the PCI bus and the VL bus are not constrained by a requirement to be backwards compatible with prior expansion bus adapters and thus provide much higher throughput than older expansion buses. Examples of devices which can be coupled to local expansion buses include SCSI adapters, network interface cards, video accelerators, audio cards, telephony cards, etc. An older-style expansion bus may also be coupled to the local expansion bus to provide compatibility with earlier-version expansion bus adapters. Examples of such expansion buses include the industry standard architecture (ISA) bus, also referred to as the AT bus, the extended industry standard architecture (EISA) bus, and the microchannel architecture (MCA) bus. Various devices may be coupled to this second expansion bus, including a fax/modem, sound card, etc.
A CPU local bus arbiter as well as a PCI bus arbiter are typically included as part of the bridge logic in many computer systems. The CPU local bus arbiter determines and prioritizes ownership of the CPU local bus, while the PCI bus arbiter determines and prioritizes ownership of the PCI bus. Mastership of either bus is typically based on a fixed arbitration fairness scheme, such as a round-robin algorithm. In some situations, a master must acquire ownership of both the PCI bus and the CPU local bus before it can proceed with a particular transfer cycle.
Computer systems were originally developed for business applications including word processing and spreadsheets, among others. Recently, computer systems have evolved toward more real time applications, including multimedia applications such as video and audio, video capture and playback, telephony, and speech recognition. Computer systems originally designed for business applications, however, are not well suited for the real time requirements of modern multimedia applications for a variety of reasons. For example, current operating systems for personal computers are not real time operating systems. In addition, the bus architecture of modern personal computer systems still presumes that the majority of applications executing on the computer system are non-real time, business applications such as word processing and/or spreadsheets which are executed solely by the main CPU.
A significant problem associated with modern computer systems is that the bus arbiter which arbitrates between real time resources, non-real time resources and the CPU is typically designed to provide the CPU with maximum availability to the system memory, or is otherwise not cognizant of the real time accesses of other bus mastering devices. Real time devices can accordingly be "starved" for memory access, particularly when a relatively large number of real time devices are included within the system. This can correspondingly result in degraded performance, unsynchronized audio and video, and the dropping of frames during video or animation sequences. Therefore, a new bus arbiter system and method are desirable which provide greater access to bus subsystems by real time devices to thus better facilitate real time applications.